In the field of optomagnetic recording media, the improvement of a recording density is one of the most important technical subjects. As means for improving the recording density of an optomagnetic recording medium is hitherto proposed a system in which a signal is recorded with multi values. The multi-valued recording system is disclosed by, for example, 13th-Lecture Abstracts of the Society of Applied Magnetism of Japan (1989), p. 63 or Japanese Journal of Applied Physics, Vol. 28 (1989) Supplement 28-3, pp. 343-347.
In the known multi-valued recording system, a plurality of magnetic layers having different coercive forces are laminated and the intensity of a magnetic field applied to the magnetic layers is modulated multi-stepwise to invert the magnetization of a specified magnetic layer selectively. It is said that according to this system, the 4-valued recording of a signal becomes possible with the provision of three magnetic layers having different coercive forces.
In the known multi-valued recording system, however, when an optomagnetic recording medium is irradiated with a laser beam at the time of recording of a signal so that the temperature of each magnetic layer is raised up to the vicinity of its Curie temperature, there is little difference in coercive force between the magnetic layers. Therefore, it is actually difficult to cause the selective inversion of the magnetization of each magnetic layer. Even if the selective inversion of the magnetization of each magnetic layer is possible in a laboratory level by strictly adjusting the magnetic characteristic of each magnetic layer while strictly controlling a laser intensity and an external magnetic field intensity at the time of recording, the mass production of such optical recording media and recording/reproducing apparatuses is impossible from the aspect of cost. Also, since margins for changes in laser intensity and external magnetic field intensity at the time of recording become remarkably small, it is impossible to maintain a stable recording/reproducing state for a long time and hence there is no practicability by any possibility. If the recording of a signal is performed in a state in which the temperature of each magnetic layer is not raised up to the vicinity of the Curie temperature or a difference in coercive force between the magnetic layers is sufficiently large, the above inconvenience will not be encountered but a large magnetic field is necessitated for the recording and erasion of a signal. This brings about another serious inconvenience that a magnetic field generating device such as a magnetic head and hence a recording/reproducing apparatus become large in size and a power consumption is increased. In this case too, therefore, the practisation is actually impossible.
In the following, the inconveniences of the prior art will be explained in more detail on the basis of FIGS. 142A, 142B, 142C and 142D. For facilitating the explanation, the following explanation will now be made taking as an example an optomagnetic recording medium in which two magnetic films (or magnetic layers) having their coercive force versus temperature characteristics represented by symbols A and B in FIG. 142B are laminated on a substrate.
(1) The temperature of a portion irradiated with a recording laser beam is raised up to a temperature equal to or higher than the Curie temperature of each magnetic layer or in the vicinity thereof. Therefore, even if a difference in coercive force between the magnetic layers is large at the room temperature, this difference becomes remarkably small at the temperature-raised portion, as shown in FIG. 142B. Accordingly, it is actually difficult to cause the selective inversion of the magnetization of each magnetic layer.
(2) The minute area of a portion irradiated with a recording laser beam has a sharp temperature distribution which extends between the room temperature and a temperature equal to or higher than the Curie temperature, as shown in FIG. 142A. Accordingly, a coercive force distribution of each magnetic layer in the corresponding area is also sharp, as shown in FIG. 142C. Therefore, whatever magnitude the set value of an applied magnetic field takes, the size of a recording domain has only an insignificant change and hence it is not possible to effect separate recording for the two magnetic layers by virtue of the magnitude of the applied magnetic field.
(3) The carrier-to-noise ratio of a signal read from each of the magnetic layers A and B for the intensity of an external magnetic field at the time of recording is as shown in FIG. 142D. Namely, a transition region between an unrecorded region and a recorded region in the case of the magnetic layer A and that in the case of the magnetic layer B nearly overlap each other in regard to the intensity of the external magnetic field and hence the carrier-to-noise ratio of the signal read from the magnetic layer A and the carrier-to-noise ratio of the signal read from the magnetic layer B have only a slight shift from each other due to a difference in leakage magnetic field to a recorded portion which difference is caused from a difference in magnetization between the magnetic layers A and B. Accordingly, the optomagnetic recording medium including the deposition of the magnetic layers A and B has only one stable recording state as shown in FIG. 142E by the carrier-to-noise ratio of a read signal. Therefore, the digitization of a recording signal into multi values by the change-over of an external magnetic field is impossible.
(4) Also, the prior art has an inconvenience that the direct overwriting of a signal is not possible. Namely, for example, in the optomagnetic recording medium including the two-layer deposition of the magnetic layer (or A layer) having a coercive force versus temperature characteristic A shown in FIG. 142B and the magnetic layer (or B layer) having a coercive force versus temperature characteristic B, the application of an external magnetic field having a magnitude H.sub.1 shown in FIG. 142B results in the inversion of magnetization of only the B layer, as shown in FIG. 143(b) and the application of an external magnetic field having a magnitude H.sub.2 shown in FIG. 142B results in the inversion of magnetization of both the A and B layers, as shown in FIG. 143(b). Accordingly, in the case where a state of FIG. 143(b) is to be recorded on a state of FIG. 143(c), the state of FIG. 143(b) is attainable provided that the magnetic field of H.sub.2 is once applied in an erasing direction for return to an initial state of FIG. 143(a) and the recording is thereafter performed again by applying the magnetic field of H.sub.1 in a recording direction. Thus, the direct overwriting of a signal is impossible.
(5) Further, in the case where a signal is recorded on this optomagnetic recording medium on the basis of, for example, a magnetic field modulating system, the application of a larger external magnetic field for performing the recording of a signal for a magnetic layer having a larger coercive force results in that a portion recorded by a smaller external magnetic field is necessarily formed around a portion recorded by the larger external magnetic field. This is because in a transition process until the external magnetic field reaches a predetermined value, the external magnetic field necessarily passes the value of a recording magnetic field for a magnetic layer having a smaller coercive force. Therefore, a reproduced signal having a high S/N ratio is not obtainable. Further, when the recording of a signal is made with a high density, there is a problem that the discrimination between a portion recorded by a larger external magnetic field and a portion recorded by a naturally smaller external magnetic field becomes difficult, thereby making it impossible to improve the recording density. Also, such inconveniences are similarly encountered in the case where a signal is recorded on the basis of a light modulating system.
The present invention is made to solve such deficiencies of the prior art and aims at the provision of an optomagnetic recording medium in which a recording state corresponding to each value in multi-valued recording can exist stably in regard to an external magnetic field, direct overwriting in multi-valued recording is possible, the recording and erasion of a signal can be made by a small external magnetic field and a small-output laser and the signal recording with a high S/N and a high recording density can be realized, the provision of a multi-valued signal recording system which uses such an optomagnetic recording medium, and the provision of a magnetic head for recording/reproducing apparatus which is suitable for the multi-valued recording of a signal on such an optomagnetic recording medium.